Generally, aircraft have attached flow or unstalled flight regions and separated flow or stalled flight regions.
In the attached flow flight region, fluid (air) flowing over the flight control surfaces behaves in a predictable and expected manner and thus control surfaces can be manipulated to control the flight path and orientation of the aircraft as it flies through the air. In fixed wing aircraft, the attached flow flight region includes angles of attack (which is the angle of incidence formed between the chord line of an airfoil and the relative wind) of the wing and horizontal stabilizer that are below a stall angle of attack. The stall angle of attack is the angle of attack at which significant separation of the fluid (air) occurs over the wing of the aircraft. At the stall angle of attack, the wing no longer generates sufficient lift to maintain level flight and the fluid flowing over control surfaces (ailerons, elevators, etc.) is no longer sufficient to allow the control surfaces to generate adequate forces to control the aircraft. As a result, the control surfaces are no longer effective in controlling the aircraft's orientation and flight path. The angles of attack beyond the stall angle of attack are generally referred to as the stalled region.
Generally, it is undesirable to operate an aircraft in the stalled flight region. To preclude operation in this region, many regulatory authorities (such as the Federal Aviation Administration (FAA) in the United States) require that the subject aircraft demonstrate sufficient stall warning margin and effectiveness. To satisfy the regulatory stall warning requirements, many aircraft manufacturers employ stall warning systems. Stall warning systems provide visual, audible, and/or tactile indications to the pilot that the aircraft is approaching the stall angle of attack. Stall warning systems do not affect the pilot control of the aircraft, and as such, the pilot may elect to ignore the stall warning system and command the aircraft into the stall (or uncontrolled) flight region.
Stall protection systems, on the other hand, prevent the aircraft from entering the stalled flight region by taking control of at least some of the flight control surfaces from the pilot and actuating the flight control surfaces to maintain the aircraft in the region below the stall angle of attack. Generally, stall protection systems prevent the aircraft angle of attack from exceeding the stall angle of attack so that the wing retains predictable lift characteristics and pilot manipulation of the control surfaces remains effective, with the exception that manipulation of the control surfaces that would cause the airplane to exceed the stall angle of attack is prevented.
Aircraft that employ stall protection systems are typically certified through a Special Condition Issue Paper process (in the U.S.), since the traditional stall requirements cannot be assessed. Some regulatory agencies (such as the FAA) may give aircraft manufacturers performance relief credits for installing stall protection systems, which can result in competitive advantages during the aircraft certification process. For example, traditional operating speed margins based on stall speed in icing conditions are not required, which results in improved takeoff and landing performance. However, while existing stall protection systems prevent aircraft excursions into the uncontrolled flight region, they do not necessarily maximize aircraft performance and pilot input can actually lead to a more rapid depletion of aircraft energy than is desired. Aircraft that have implemented stall protection systems have generally removed traditional stall warning systems and replaced the stall warning demonstrations with stall robustness demonstrations.